Bump pyyaml from 3.12 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.1)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -37,7 +37,7 @@ pygments-style-solarized==0.1.1
 pylibmc==1.5.2
 python-dateutil==2.6.1
 pytz==2017.3
-PyYAML==3.12
+PyYAML==5.1
 requests==2.18.4
 simplegeneric==0.8.1
 six==1.11.0